Could you please confirm whether clause 431.1.1 (iii) in BS 7671 is accurate and up to date?

Question

As per the requirements of BS 7671 , a circuit breaker may take up to one hour to trip when subjected to a current of approximately 1.45 times its rated current . In such cases, we would like to clarify whether the associated cable is also designed to withstand 1.45 times its continuous current-carrying capacity for the same duration without degradation or thermal damage. 
 Is this aspect covered in any cable standards (such as BS 6004 , IEC 60287 , or IEC 60364 ) or specified in cable manufacturer datasheets ? 
 We seek clarification on whether:

The cable&rsquo;s short-term thermal endurance aligns with the breaker&rsquo;s tripping characteristics.

Manufacturers consider this 1.45 &times; In scenario when defining safe operational limits.

Any references or guidelines from standards or datasheets that confirm the cable's ability to tolerate such overloads for a limited duration would be helpful.

gkenyon · Accepted Answer

BS 7671:2018+A2:2022+A3+2023 is the latest version of the standard. 
 In the latest version, Regulation 431.1.1 does not have an indent (iii):

Do you mean Regulation 433.1.1? 
 In terms of the question: we would like to clarify whether the associated cable is also designed to withstand 1.45 times its continuous current-carrying capacity for the same duration without degradation or thermal damage. 
 The key to understanding this requirement is that I z is the current-carrying capacity of a cable for continuous service under the particular installation conditions concerned . For determination of I z , see Appendix 4 to BS 7671. Where the cable types (or conditions) are not covered in Appendix 4, other approaches are not precluded, including BS EN 20287 series and BS 7769 series.

gkenyon · Answer

Thank you, in that case, the following answer I gave is relevant: 
 gkenyon said: In terms of the question: we would like to clarify whether the associated cable is also designed to withstand 1.45 times its continuous current-carrying capacity for the same duration without degradation or thermal damage. 
 The key to understanding this requirement is that I z is the current-carrying capacity of a cable for continuous service under the particular installation conditions concerned . For determination of I z , see Appendix 4 to BS 7671. Where the cable types (or conditions) are not covered in Appendix 4, other approaches are not precluded, including BS EN 20287 series and BS 7769 series.

gkenyon · Answer

Sanjith Mohan said: Can cables reliably withstand 1.45 times their rated continuous current 
 Yes ... but not for an indefinite time. Hence, also the requirement of Regulation 433.1 'Every circuit shall be designed so that small overload of long duration is unlikely to occur'. 
 The design element that is intended to ensure that the cable is not overloaded is covered in the first two conditions of 433.1.1, i.e. 433.1.1(i) and 433.1.1 (ii), which states, in effect: 
 I b &le;I n &le;I z 
 The missing piece of the puzzle lies between I n and a full blown fault current (Section 434). For protection against fault current in general, Regulation 434.5 applies ... Regulation 434.5.2 makes the assumption (because the short duration of the high current fault), that no heat is lost from the outside of the cable during the fault ... we call that an 'adiabatic approach'. For faults currents that take over 5 seconds or so to disconnect, the estimates of the adiabatic approach no longer apply, and more current can be handled than the formula in 434.5.2 shows. We can then use the 'non-adiabatic approach' explained in BS 7454 to calculate what the cable can withstand.

mapj1 · Answer

It is there because you have to have a gap between the current that any real breaker can carry all day without tripping - it's full load current, and a higher value, the highest current it might not trip at. Above that are current levels at which it certainly will operate, with increasing speed with increasing currents. That 1.45 figure is the width of that uncertain region. So a 32A breaker can take 32A all day , at full temperature. It is certain to trip within 1 hour at around 40A, but between 32A and 40A is a grey area, where what it does will depend on the day, and where in the natural scatter of the production line that one particular breaker sits. The cable rating is a function of cable size, its location, grouping with other cables etc, and is a matter for the designer to allow for this - normally you'd not put a 40A load on a 32A breaker for example. Mike. 
 PS cables can take massively more than their continuous rating for short periods, it is the race between the cable heating from the overload and cooling to the environs - and at what temperature a final equilibrium is reached.

gkenyon · Answer

gkenyon said: We can then use the 'non-adiabatic approach' explained in BS 7454 to calculate what the cable can withstand. 
 You can read more about this approach in Section 6.5 of the IET Commentary on the IET Wiring Regulations . Section 6.7 of the same publication goes on to discuss the application of the techniques for flexible cables. 
 Regulation 433.1.1 is also explained on Page 28 of the IET's Guidance Note 6: Protection against overcurrent , which states: 
 Condition (iii) provides the overload protection required by Regulation 433.1.1, but note that it is concerned primarily with the conductors. Protection for electrical equipment in the circuit, including the overload device itself, should be covered by careful selection of equipment, made to a standard quoted in BS 7671 or to an equivalent. 
 The factor 1.45 in condition (iii) is based on a combination of experience and investigation. This has shown that the types of cable considered in BS 7671 can safely withstand a small but undefined number of periods at excess temperatures corresponding to currents not greater than 1.45 times their current-carrying capacity. However, such currents must not persist for long periods.

Chris Pearson · Answer

gkenyon said: Hence, also the requirement of Regulation 433.1 'Every circuit shall be designed so that small overload of long duration is unlikely to occur'. 
 How long is a piece of string? 
 Or in this case, how small is, "small"; and what amounts to a, "long duration"? 
 Clearly they are inversely related, but a "small" overload could be 45% (but if your bank made an error of 45%, it would hardly be small). I regularly overload a BS 1363 socket when I turn on my toaster and kettle each morning, but 3 minutes is hardly a "long duration". 
 Back to the OP, I have always assumed that if you take the tables in Appendix 4, the CCC takes into account that there could be a small overload of long duration, but I may be wrong. 
 So, Table 4D2A, a 6 mm&sup2; 2-core cable installed according to reference method A should have a 32 A MCB (maximum). Just as the MCB will tolerate 46 A for a long duration, so will the cable.

mapj1 · Answer

How long is a piece of string? Or in this case, how small is, "small"; and what amounts to a, "long duration"? 
 It need not be quite as imprecise as it at first seems; although it is well hidden. Once can look at the adabatic curves of cable current handling, and the constant current ratings and look at the intersection. The answer is that the time for a cable to reach equilibrium in free air case for most common domestic cables is a few minutes. 
 The second aspect is my how much the temperature rating of a cable can be exceeded, and what impact this has on the life of the insulation. 
 You can drop an offcut of cable into your mates cup of tea, and despite being well over 70c, the cable does not melt away like butter, it just gets a bit extra flexible. 
 While it has now disappeared from the IET website, way back in 2004. it contained this table . While assuming all PVC is created equal is rather dodgy, it does allow some estimation of the life shortening effect of overheating - and as temperature rise is generally proportional to the square of current, also tolerance to overloads (a current increase of 1.41 times will at worst double the temperature rise, perhaps less where convection cooling occurs and air moves faster with rising temperature). In summary a hour or two at up to perhaps 110C for a cable designed for operation of many decades at 70C is not likely to hurt it. The corollary, is that by designing things that run cool, they will probably outlive the building, let alone the installer. regards Mike.

AJJewsbury · Answer

On what basis was this factor of 1.45 established? 
 
 I think it goes back a long way. If you look though the older versions of the wiring regs (long before they became BS 7671) they contained what was in effect the specification and test procedures for cables - only in more recent decades have such things been split out into specific cable standards. 
 As others have said, 1.45 is probably down to ' experience and investigation ' rather than any particular calculation. The concept of allowing conductors to run "too hot" for a limited time isn't unique to this situation though - during faults we allow conductors to get really very hot (have a look at the "final temperature" row on table 43.1 in BS 7671 for one example). The point is that cables don't suddenly self destruct or catch fire the moment they're overloaded, but rather they run rather warmer and the insulation degrades rather faster that it would under normal conditions - but given that in many situations insulation will be good for 70 years or more, a few events that perhaps knock a few months off the lifespan are likely to be acceptable. 
 - Andy.

Could you please confirm whether clause 431.1.1 (iii) in BS 7671 is accurate and up to date?

Community Rules & Guidelines